MID-V3 Phase 0-2: Design doc, type skeleton, and RegionIdBox API

- MID-V3-0: Create design doc (docs/analysis/MID_POOL_V3_DESIGN.md)
  - Lane vs Page role clarification
  - Phase plan and checklist

- MID-V3-1: Type skeleton + ENV
  - MidHotBoxV3, MidLaneV3, MidPageDescV3 structures
  - ENV controls (HAKMEM_MID_V3_ENABLED, HAKMEM_MID_V3_CLASSES)
  - Cold interface declarations

- MID-V3-2 (V6-HDR-2): RegionIdBox Registration API completion
  - RegionEntry structure with sorted array storage
  - Binary search lookup implementation
  - region_id_register_v6() / region_id_unregister_v6()
  - REGION_KIND_MID_V3 added to enum

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

core/box/mid_cold_iface_v3.h

@@ -0,0 +1,110 @@
+// mid_cold_iface_v3.h - Cold Interface for Mid/Pool v3
+//
+// 役割:
+//   - L1 レベルの refill / retire 操作を定義
+//   - Page carve / return
+//   - RegionIdBox 登録 / 解除
+//   - Phase MID-V3-1: インターフェース宣言のみ
+#pragma once
+
+#include <stddef.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "mid_hotbox_v3_box.h"
+
+// ============================================================================
+// Cold Interface Function Pointers (for future vtable support)
+// ============================================================================
+
+typedef MidPageDescV3* (*mid_cold_refill_fn)(MidHotBoxV3* hot, uint32_t class_idx);
+typedef void (*mid_cold_retire_fn)(MidHotBoxV3* hot, MidPageDescV3* page);
+typedef bool (*mid_cold_remote_push_fn)(MidPageDescV3* page, void* ptr, uint32_t tid);
+typedef void (*mid_cold_remote_drain_fn)(MidHotBoxV3* hot);
+
+typedef struct MidColdIfaceV3 {
+    mid_cold_refill_fn      refill_page;
+    mid_cold_retire_fn      retire_page;
+    mid_cold_remote_push_fn remote_push;
+    mid_cold_remote_drain_fn remote_drain;
+} MidColdIfaceV3;
+
+// ============================================================================
+// Default Cold Interface (direct function calls)
+// ============================================================================
+
+/// Refill: Get a new page from segment/pool
+/// @param hot: TLS HotBox context
+/// @param class_idx: Size class index
+/// @return: New page with freelist, or NULL on failure
+MidPageDescV3* mid_cold_v3_refill_page(MidHotBoxV3* hot, uint32_t class_idx);
+
+/// Retire: Return an empty page to segment/pool
+/// @param hot: TLS HotBox context
+/// @param page: Page to retire (must be empty)
+void mid_cold_v3_retire_page(MidHotBoxV3* hot, MidPageDescV3* page);
+
+/// Remote push: Push freed block to remote thread's page
+/// @param page: Target page (owned by different thread)
+/// @param ptr: Block to free
+/// @param tid: Caller's thread ID
+/// @return: true if push succeeded, false if local handling needed
+bool mid_cold_v3_remote_push(MidPageDescV3* page, void* ptr, uint32_t tid);
+
+/// Remote drain: Process blocks freed by other threads
+/// @param hot: TLS HotBox context
+void mid_cold_v3_remote_drain(MidHotBoxV3* hot);
+
+// ============================================================================
+// Segment Operations
+// ============================================================================
+
+/// Acquire a segment for the given class
+/// @param class_idx: Size class index
+/// @return: Segment pointer, or NULL on failure
+MidSegmentV3* mid_segment_v3_acquire(int class_idx);
+
+/// Carve a page from segment
+/// @param seg: Segment to carve from
+/// @param class_idx: Size class index
+/// @return: New page descriptor, or NULL if segment exhausted
+MidPageDescV3* mid_segment_v3_carve_page(MidSegmentV3* seg, int class_idx);
+
+/// Return a page to segment
+/// @param seg: Owning segment
+/// @param page: Page to return
+void mid_segment_v3_return_page(MidSegmentV3* seg, MidPageDescV3* page);
+
+// ============================================================================
+// Lane Operations
+// ============================================================================
+
+/// Refill lane from page
+/// @param lane: TLS lane
+/// @param page: Source page
+/// @param batch_size: Number of items to transfer
+void mid_lane_refill_from_page(MidLaneV3* lane, MidPageDescV3* page, uint32_t batch_size);
+
+/// Flush lane back to page
+/// @param lane: TLS lane
+/// @param page: Target page
+void mid_lane_flush_to_page(MidLaneV3* lane, MidPageDescV3* page);
+
+// ============================================================================
+// Page Operations
+// ============================================================================
+
+/// Build freelist for a page
+/// @param page: Page descriptor
+/// @return: Freelist head
+void* mid_page_build_freelist(MidPageDescV3* page);
+
+/// Push a block to page's freelist (thread-safe)
+/// @param page: Target page
+/// @param ptr: Block to push
+void mid_page_push_free(MidPageDescV3* page, void* ptr);
+
+/// Pop a block from page's freelist
+/// @param page: Source page
+/// @return: Block pointer, or NULL if empty
+void* mid_page_pop_free(MidPageDescV3* page);